Photochemistry of Sulfur Dioxide Mediated by Charge Transfer through Intermolecular Chalcogen Bonds

The photochemistry of sulfur dioxide (SO2) plays a crucial role in the Earth's atmosphere. In contrast to the well-established photochemistry of free SO2, the electronic properties and reactivity of SO2 complexes with other atmospherically relevant gas molecules are barely known. Herein, we demonstrate that the photochemistry of SO2 can be dramatically influenced by the first formation of charge-transfer (CT) prereaction complexes through intermolecular chalcogen bonds. Specifically, the prototype sulfur-sulfur bonded complex between SO2 and hydrogen sulfide (H2S) displays a strong CT absorption at 290 nm, and photoexcitation of the complex at this CT band furnishes the elusive thiosulfurous acid (HSS(O)OH), which is a missing intermediate in the Claus sulfur recovery process. Similar charge-transfer mediated photochemistry is also applicable to the chalcogen-bonded complexes of SO2 with the atmospherically important volatile organic sulfur compounds dimethylsulfide and methanethiol, from which sulfinic acids form efficiently through photolytic SO2-insertion along with concerted hydrogen atom transfer. This work highlights the pivotal role of intermolecular chalcogen bonds influencing the photochemistry of SO2 due to the formation of photolabile charge-transfer complexes as prereaction intermediates.